68 PROCEEDINGS OP THE NATIONAL MUSEUM vol.76 



fidence in the faunal and stratigraphic criteria of diastrophism, 

 which in my definition of the term includes evidence of any move- 

 ment of the surface of the earth that occasioned displacement of the 

 marine strandline and resulted in generic modifications of the pre- 

 ceding composition of faunas and floras or in their local extinction 

 or complete replacement by faunas derived from other sources. My 

 confidence has grown constantly also in the consequent paleontologi- 

 cal principle that the more or less abrupt introduction of new generic 

 types in epicontinental marine deposits, especially if the new elements 

 were derived from a previously excluded faunal realm, is a far more 

 reliable criterion in fixing stratigraphic boundaries and in determin- 

 ing their taxonomic significance than is the general or composite 

 aspect of the fauna in which these foreign- constituents occur. The 

 importance of these invasions of foreign elements — whether they 

 appear but once or repeatedly at intervals in a given section — lies in 

 the probable fact that some diastrophic movement, the results of 

 which included submergence of a previously excluding barrier, had 

 occurred at times shortly preceding their advent. 



That my confidence in these views has not been misplaced is 

 proved by the great success that has attended their application in 

 American stratigraphic problems. As they served very well in these 

 it seemed probable that they would serve equally well in the appar- 

 ently quite similar European cases in question. Accordingly, in the 

 paper on the Ordovician-Silurian boundary already cited I advo- 

 cated removal of the Drummuck, Keisley, Upper Leptaena, and other 

 north European formations generally regarded as of approximately 

 like age from the lower side of the line to above it. Obviously, it is 

 with considerable interest that I await the reaction of European 

 opinion to my proposal. For two reasons the points mainly at issue 

 are presented here again, though more briefly and viewed from some- 

 what different angles; first, because the opportunity to say what I 

 hope may be my last word on the subject is at hand; and, second, be- 

 cause the facts mentioned in its discussion have a decided bearing 

 also on my contention respecting the generic persistence of the indig- 

 enous life of the middle Atlantic realm — or, indeed, of any of the 

 centers of faunal development and dispersal — which I find to be 

 much greater than anyone believed heretofore. It has been also 

 the most pregnant though perhaps least suspected cause of error and 

 general confusion in correlating formations of different provinces. 



General similarity of fossil contents without precisely identified 

 closely drawn species certainly does not establish contemporaneity 

 of the compared deposits. On the contrary, it usually indicates dif- 

 ference in age. Moreover, it is practically impossible to correctly 

 estimate the chronologic significance of such differences except when 



